1. Field of the Invention
The present disclosure relates to a positive electrode active material for non-aqueous secondary battery and method of manufacturing the same.
2. Description of Related Art
The non-aqueous secondary batteries typified by lithium ion secondary batteries employ materials which allow desorption/insertion of alkali metal ions for positive electrode active materials, and single alkali metals such as metallic lithium or materials which allow desorption/insertion of alkali metal ions for negative electrode active materials, and non-aqueous electrolytic solutions for alkali metal ion conductive materials. In such non-aqueous secondary batteries, alkali metal ions are transferred between positive and negative electrodes through the alkali metal ion conductive materials to supply electric power to an external load. In a lithium ion secondary battery, a lithium transition metal composite oxide such as lithium cobaltate is typically used as a positive electrode active material.
A lithium ion secondary battery is one type of non-aqueous electrolyte secondary batteries, in which a non-aqueous electrolyte solution obtained by dissolving an electrolyte, which contains lithium ion, in an organic solvent is used as a lithium ion conductive material. As described above, non-aqueous secondary batteries employ an organic solvent and thus inherently dangerous because of the combustible and flammable nature or the like, requiring fire or explosion prevention measures.
Meanwhile, another type of lithium secondary battery is all-solid lithium secondary batteries which employ a lithium ion conductive inorganic solid substance (solid electrolyte) for a lithium ion conductive material. The need of an organic solvent can be eliminated in such all-solid secondary batteries such as all-solid lithium secondary batteries, so that safety measures in the non-aqueous electrolyte secondary batteries can be dispensed with, allowing for much simpler configurations of the batteries.
However, the all-solid secondary batteries generally have output characteristics lower than that of non-aqueous electrolyte secondary batteries. One of the causes is thought that a high-resistance region created in the interface between the solid electrolyte and electrode active material suppresses movement of the lithium ions. In order to improve the interface between the positive electrode active material and other components, it is proposed to cover the surface of the positive electrode active material with a specific material. Examples of the covering material include a niobium compound.
In JP 2011-070789A, proposed is a technology of adding niobium to a lithium-containing transition metal composite oxide in which nickel and manganese are essential components and which has a layered structure. It is said in JP 2011-070789A, according to the technology, the interface between the positive electrode and the non-aqueous electrolyte is improved and charge transfer reaction is accelerated, and output characteristics can be improved. As for more specific example of a method to add niobium, a method of mixing a lithium-containing transition metal composite oxide and niobium oxide at a predetermined mixing ratio and sintering the mixture at a predetermined temperature is disclosed.
In WO 2007/004590A, proposed is a technology of covering a surface of a positive electrode active material in an all-solid lithium secondary battery which employs a sulfide-based solid electrolyte. It is said in WO 2007/004590A, according to the technology, generation of a high-resistance layer in an interface between the sulfide-based solid electrolyte and the positive electrode active material can be suppressed and output characteristics of the all-solid lithium secondary battery can be improved. As for an example of the lithium ion conductive oxide, LiNbO3 is illustrated. As for an example of more specific covering method, disclosed is a method in which an alkoxide solution which contains lithium and niobium is sprayed to particles of a positive electrode active material and then hydrolyzed by the moisture in the air. As for an example of the positive electrode active material, LiCoO2 and LiMn2O4 are illustrated.
In JP 2004-253305A, proposed is a technology of adding a niobium compound or the like to a surface of a lithium nickel composite oxide, and sintering. It is said in JP 2004-253305A, according to the technology, the niobium compound or the like can be present stably on the surface of the lithium nickel composite oxide, so that the niobium compound or the like on the surface can be suppressed from dissolving into the electrolytic solution, which can suppress a rise of impedance during storage at a high temperature and a cycle operation at a high temperature. More specifically, disclosed is a method in which a lithium nickel composite oxide is dispersed in a commercial niobium oxide sol dispersed in acetone and then the acetone is evaporated, and the remained mixture is heated at a temperature of 120° C. to solidify the mixture. In JP 2004-253305A, the dispersion medium of the commercial niobium oxide is not described.
As for the material which contains niobium, a niobium oxide sol is known. For example, JP H06-321543A describes that a niobium oxide sol which contains oxalic acid and a niobium oxide has a fine particle diameter of 100 angstrom or less yet it is stable at a (HCOO)2/Nb2O5 molar ratio of 0.2 to 0.8. It is said in JP H06-321543A, such a niobium oxide sol can be obtained by adding a predetermined amount of oxalic acid into an active niobium hydroxide slurry and conducting a thermal reaction under predetermined conditions.
In JP H08-143314A, described is a niobium oxide sol which is obtained by adding citric acid to an oxalic acid-stabilized niobium oxide sol can be present stably in the presence of other metallic elements such as cobalt.